The previous owner said that he had tried a new CPU, cleaning all of the tarnish of the chips and the 74LS139 and 74LS11 associated with the video RAM address decoding.

He’s clearly better at this than I am but I’m still going to try and get this old one going.

First things first

Out with the multimeter and check the power supply. There are three rails; 5V, -5V, 12V and -12V. Helpfully there are four LEDS on the PSU board and they are all lit up. Also, the meter confirms that all are present and correct.

The bits on the bus go up and down

Sadly actually most of them don’t. I fired up the oscilloscope and looked at the signals on the pins of the Z80 CPU. There is very little activity. There is a clock signal on pin 6. MREQ is toggling. There is little else. There is no activity on M1 that signals the start of an instruction. The data pins are all high as are the address pins.

Surely there should be activity on the address bus?

That’s it for Today. I will read up on bus access and try to see why the address lines aren’t changing.